Self-compensating blood pressure bleed valve

ABSTRACT

A bleed flow valve for venting air from a blood pressure cuff includes a valve body having an axially extending central bore. The central bore has a transition section, a fluid inlet port opening into the central bore upstream of the transition section and a fluid outlet port opening into the central bore downstream of the transition section. A piston is disposed within the central bore of the valve body and is axially translatable within the central bore of the valve body. An annular orifice is defined between an outer circumferential surface on the piston and the transition section of the central bore. The bleed valve is self-compensating as the pressure within the cuff decreases in that the piston self adjusts axially within the central bore to adjust the area of the annular orifice so as to maintain a relatively constant cuff pressure change rate throughout the deflation process.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of blood pressure measurement devices and, more particularly, to a nearly constant flow rate bleed valve for use in either manual or automatic blood pressure measurement devices.

Blood pressure measurement devices, also referred to as sphygmomanometers, of the type commonly used to measure arterial blood pressure include an inflatable sleeve, commonly referred to as a cuff, adapted to fit around a limb, e.g. an arm or leg, of a patient. The cuff includes an interior chamber that is in fluid communication with a device for selectively inflating the interior chamber of the cuff with pressurized air. A gage is operatively connected in fluid communication with the interior chamber of the cuff for monitoring the air pressure within the cuff. A bleed valve is also operatively connected in fluid communication with the interior chamber to permit selective depressuring of the interior chamber when it is desired to deflate the cuff.

In conventional manual sphygmomanometers, the interior chamber of the cuff is connected through a length of flexible tubing to a pneumatic bulb. In operation, the cuff is fitted, e.g. wrapped, about the arm of the patient and, once so positioned; the cuff is inflated by squeezing the pneumatic bulb to force air through the tubing into the interior chamber of the cuff. Once the interior chamber of the cuff has been inflated to a desired level, as indicated on the pressure gage, the cuff is deflated by opening the bleed valve to allow the pressurized air within the interior chamber of the cuff to vent to atmosphere. A stethoscope is positioned under the cuff and over the patient's artery to monitor the patient's arterial pulses as the cuff deflates, thereby allowing the systolic and diastolic blood pressures to be determined by listening for the Korotkoff sounds. It can also be done oscillometrically by detecting the minute changes in the cuff pressure due to flow through the brachial artery.

Electronic oscillometric blood pressure measurement devices that utilize an inflatable cuff are also known in the art. Such devices generally employ one or more pressure sensing devices, such as a transducer, to monitor the pressure within the interior chamber of the cuff, as well as the minute changes in the cuff pressure due to flow in the patient's artery, as the cuff deflates. Electronic circuitry is provided that processes the signals from the pressure sensing devices and determines the systolic and diastolic blood pressures. A motor driven pump is usually provided to inflate the cuff. However, the inflation can be done by a pneumatic bulb. Typically, a digital display is provided for displaying the systolic and diastolic blood pressures.

To obtain accurate measurements, it is necessary to deflate the inflated cuff at a relatively constant rate in the range of about 2 to about 3 millimeters mercury (2-3 mmHg) per second or about 2 to about 3 millimeters mercury (2-3 mmHg) per heartbeat. Maintaining a relatively constant bleed flow rate has been a problem when using many prior art sphygmomanometers, particularly when used by untrained personnel. In prior art sphygmomanometers, the bleed commonly comprises a fixed orifice vent valve, that is a valve which vents the interior chamber of the inflated cuff through a fixed area port. With a fixed area port, the vent flow rate varies as a function of the pressure differential across the port at any given time in the venting process. As the pressure within the interior chamber will continuously decrease during the deflation process, the pressure differential, that is the difference between the air pressure within the interior chamber of the cuff and ambient pressure, will also continuously decrease. Therefore, as the pressure differential across the vent port is continuously decreasing, the vent flow rate will not remain relatively constant during the deflation process, but rather will continuously decrease. U.S. Pat. No. 4,690,171, for example, discloses a bleed valve having a fixed air bleed orifice assembly for metering the vent flow to slowly deflate the cuff and a separate opening for rapid deflation of the cuff.

Constant-rate deflation valves using a flexible valve member to vary the effective vent port area are known in the art for use in connection with sphygmomanometers. For example, U.S. Pat. No. 5,833,620 discloses a constant rate deflator including a ventilation adjusting shaft that has a recess formed on one end thereof, and a ventilation valve having a hole formed at the center thereof and a projection having almost the same shape as that of the recess in the ventilation adjusting shaft. The ventilation valve is made of a flexible material, such as rubber, whereby the width of the clearance between the projection from the ventilation valve and the valve recess increases as the pressure of the air venting through the valve decreases such that the flow rate through the clearance remains relatively constant. U.S. Pat. No. 5,143,077 also discloses a constant rate discharge valve for a sphygmomanometer utilizing a flexible valve body to move a valve stem to control vent port size in response to the pressure of the fluid venting therethrough.

For proper functioning in controlling the rate of vent flow, such constant-rate valves rely upon a predictable response of the flexible valve member to changes in pressure differential across the flexible member. Over repeated flexing, the potential exists for such flexible members to lose some degree of flexibility and even to crack or otherwise fail from fatigue after repetitive flexure under pressure.

SUMMARY OF THE INVENTION

It is an object of one aspect of the invention to provide a self-compensating bleed valve exhibiting a relatively constant bleed flow rate.

It is an object of one aspect of the invention to provide a relatively constant flow rate bleed valve that does not employ any flexible diaphragm to adjust the flow area of vent port in response to varying pressure differential across the vent port.

It is an object of one aspect of the invention to provide a bleed flow valve for use in connection with deflating a blood pressure cuff.

It is an object of one aspect of the invention to provide a method of deflating a blood pressure cuff.

In one aspect of the invention, a bleed flow valve is provided for controlling the pressure change rate in a reservoir of fluid under pressure when venting fluid therefrom through the valve. The bleed valve includes a valve body having a central bore extending axially therethrough. The central bore has a transition section, a fluid inlet port opening into the central bore upstream of the transition section and a fluid outlet port opening into the central bore downstream of the transition section. The transition section may be provided by a step change from a larger diameter upstream cavity to a smaller diameter downstream cavity or by an inwardly tapered surface extending between a larger diameter upstream cavity and a smaller diameter downstream cavity. A piston is disposed within the central bore of the valve body and is axially translatable within the central bore of the valve body. The piston includes an outer circumferential surface facing the transition section of the central bore. An annular orifice is defined between the outer circumferential surface on the piston and the transition section of the central bore. A biasing device operatively associated with the piston exerts a force on the piston acting to translate the piston in an upstream direction in opposition to a fluid pressure force on the piston acting to translate the piston in a downstream direction. In this manner, the bleed valve of the invention is self-compensating as the pressure within the reservoir decreases in that the piston self adjusts axially within the central bore to adjust the area of the annular orifice so as to maintain a relatively constant pressure change rate in reservoir pressure throughout the venting process.

In one embodiment, the biasing device comprises a spring member and a preload device for compressing the spring member to establish an initial preload force on the piston acting to translate the piston in an upstream direction. The spring member may be a compressible coil spring disposed about a downstream portion of the piston. The preload device may be a closure member disposed within the central bore downstream of and abutting the spring member, the closure member being selectively axially positioned so as to adjust the preload force on the piston.

In an embodiment, the outer circumferential surface on the piston may be a tapered outer circumferential surface. In another embodiment, the outer circumferential surface may be provided on a circumferential ridge extending about the piston. In another embodiment, a circumferential ridge having an tapered outer circumferential surface may be provided on the piston.

In an embodiment, a first proximal closure member closes the first end of the valve body and a second distal closure member closes the second end of the valve body. Further, a low pressure stop that is selectively axially translatable may be supported by the first closure member and a high pressure stop that is selectively axially translatable may be supported by the second closure member.

In a further aspect of the invention, a bleed flow valve is provided for controlling the pressure change rate experienced by a flow of air under pressure venting from a blood pressure cuff. The valve includes an axially elongated valve body having a first end, a second end, and a central bore extending axially therethrough. The central bore has a first generally cylindrical cavity, a second generally cylindrical cavity, and third cavity disposed between the first and second cavities with the third cavity having a tapered transition section. A first closure member closes the first end of the valve body and a second closure member closes the second end of the valve body. A piston is disposed within the central bore of the valve body. The piston has a generally cylindrical configuration and includes a tapered outer circumferential surface facing the tapered transition section of the central bore, thereby defining an annular orifice between the tapered outer circumferential surface on the piston and the tapered transition section of the central bore. The piston is axially translatable within the central bore of the valve body. A first port, which opens through the valve body into the first cavity upstream of the tapered outer circumferential surface on the piston, is pneumatically coupled to the blood pressure cuff. A second port, which opens through the valve body into the second cavity downstream of the tapered outer circumferential surface on the piston, provides a vent hole. A biasing device operatively associated with the piston exerts a force on the piston acting to translate the piston in an upstream direction in opposition to a fluid pressure force on the piston acting to translate the piston in a downstream direction. In this manner, the bleed valve of the invention is self-compensating as the pressure within the reservoir decreases in that the piston self adjusts axially within the central bore to adjust the area of the annular orifice so as to maintain a relatively constant cuff pressure change rate throughout the venting process. The piston self adjusts axially within the central bore to vary the flow area defined by the annular orifice so as to maintain a relatively constant cuff pressure change rate substantially independently of the size of the blood pressure cuff.

In one embodiment, the biasing device comprises a spring member and a preload device for compressing the spring member to establish an initial preload force on the piston acting to translate the piston in an upstream direction. The spring member may be a compressible coil spring disposed about a downstream portion of the piston. The preload device may be a closure member disposed within the central bore downstream of and abutting the spring member, the closure member being selectively axially positioned so as to adjust the preload force on the piston. Advantageously, the first closure member may be an end cap and the second end closure may be an end plug. Further, a low pressure stop that is selectively axially translatable may be supported by the first closure member and a high pressure stop that is selectively axially translatable may be supported by the second closure member.

In one embodiment, a selectively positioned vent control valve is operatively associated with the bleed valve to provide for selectively closing the second port of the bleed valve or opening the second port of the bleed valve to vent to atmosphere pressure. Advantageously, the vent control valve may include a valve body having a cavity therein having a first port opening to the cavity and in flow communication with the blood pressure cuff, a second port opening to the cavity and in flow communication with the second port of the bleed valve, and a third port opening to the cavity and venting to atmospheric pressure. A selectively positionable member is operatively associated with the valve body. The member is selectively positioned in a first position wherein neither of the first port or the second port of the vent control valve is in flow communication with the third port of the vent control valve, in a second position wherein only the second port of the vent control valve is in vent communication with the third port of the flow control valve, and a third position wherein only the first port of the vent control valve is in flow communication with the third port of the vent control valve.

In a further aspect of the invention, a method is provided for venting an inflated blood pressure cuff through a bleed valve at a relatively constant cuff pressure change rate. The method includes the steps of: providing an annular orifice having a variable flow area between an upstream cavity and a downstream cavity; passing air under pressure from the blood pressure cuff to the upstream cavity; venting air from the downstream cavity; and varying the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity so as to maintain a relatively constant cuff pressure change rate. Further, the method may include the step of preventing an increase in the flow area of the flow path between the upstream cavity and the downstream cavity in response to a decrease in the pressure of the air within the upstream cavity to a predetermined maximum flow area at a predetermined relatively low pressure. The method may also include the step of preventing a decrease in the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity to a predetermined minimum flow area at a predetermined relatively high pressure.

The method may include the step of exerting a biasing force on the piston acting to translate the piston in an upstream direction in opposition to a fluid pressure force on the piston acting to translate the piston in a downstream direction, whereby the piston self adjusts axially within the central bore to adjust the annular orifice so as to maintain a relatively constant cuff pressure change rate. The method may include the step of establishing an initial biasing force on the piston acting to translate the piston in an upstream direction, whereby a desired cuff pressure change rate is preset. The method may include the step of adjusting the biasing force on the piston acting to translate the piston in an upstream direction, whereby a desired cuff pressure change rate may be selectively set. The biasing force may be adjusted in response to a change cuff size from that for which the biasing force was preset; or in response to heart rate of the patient being significantly different from the normal heart rate for which the biasing force was preset.

In another aspect, a method is provided for automatically varying the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity so as to maintain said relatively constant cuff pressure change rate substantially independently of blood pressure cuff size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation view of a particular embodiment of a bleed flow valve in accordance with the present invention;

FIG. 2 is an exploded elevation view, partly in section, of the end cap and low pressure stop assembly of the bleed flow valve of FIG. 1;

FIG. 3 is an exploded elevation view, partly in section, of the end plug and high pressure stop assembly of the bleed flow valve of FIG. 1;

FIG. 4 is a cross-section view taken along line 4-4 of FIG. 3;

FIG. 5 is an exploded side elevation view, partly in section, of the valve body and piston of the bleed flow valve of FIG. 1;

FIG. 6 is a cross-section view taken along line 6-6 of FIG. 5;

FIG. 7 is an enlarged side elevation view of the conical transition section of the valve body of the bleed flow valve of FIG. 1 illustrating the piston in a first operational position and a second operational position; and

FIGS. 8A, 8B and 8C are elevation views, partly in section, of the bleed flow valve of FIG. 1 connected in operative association with a three position vent control valve, with the vent control valve in its first, second and third positions respectively.

DETAILED DESCRIPTION

Referring now to FIGS. 1 through 6, the bleed valve 10 includes an axially elongated body 30 defining a centrally disposed and axially elongated central bore 32, closed at one end by a first end fitting supporting a low pressure stop 42, and closed at its other end by a second end fitting supporting a high pressure stop 52. A first set of threads 31 is provided on the exterior surface of the body 30 at one end thereof and a second set of threads 33 is provided on the surface of the bore 32 at the other end of the body 30 of the bleed valve 10. As will be discussed in further detail hereinafter, the central bore 32 of the body 30 has a central cavity 35 disposed between a pair of axially spaced, larger diameter end cavities 37 and 39.

In the embodiment shown, the first end fitting comprises an end cap 40 provided with a set of threads 41 on an internal end bore 43 that are compatible with the first set of threads 31 on body 30 whereby the end cap 40 may be releaseably mounted to the body 30 by screwing the set of threads 41 onto the set of threads 31. A circumferential seal 91, for example an O-ring seal, is disposed in the bore 43 of the end cap 40 intermediate the body 30 and the end cap 40 and is supported in a circumferential groove 92. The circumferential groove 92 may be formed in the exterior surface of the body 30, as shown in the depicted embodiment, or formed in the surface of the bore 43 in the end cap 40.

Additionally, the end cap 40 has a central bore 45 in its end surface which extends to and opens into the bore 43 of the cap 40. The low pressure stop 42 has a head 44, a shaft 46 extending from the head 44, and a tip 48 that extends axially from the shaft 46 into the bore 32 of the body 30. A circumferential seal 93, for example an O-ring seal supported in a circumferential groove 94, is disposed in the central bore 45 of the end cap 40 to seal the clearance between the surface of the bore 45 in the end cap 40 and the shaft 46 of the low pressure stop 42. The circumferential groove 94 may be formed in the surface of the low pressure stop 42, as shown in the depicted embodiment, or formed in the surface of the central bore 45. The tip 48 has threads 47 by means of which the low pressure stop 42 may be threaded into threads 49 provided in the surface of the passage between the central bore 45 and the end bore 43 of the end cap 40. Advantageously, the low pressure stop 42 may be selectively axially adjustable within the central bore 45 by screwing or turning the head 44 of the low pressure stop 42 either clockwise or counter clockwise thereby selectively positioning the tip 48 of the low pressure stop 42 within end chamber 37 of the bore 32 of the body 30.

In the embodiment shown, the second end fitting comprises an end plug 50 provided with a set of threads 51 on its outer surface that are compatible with the second set of threads 33 on body 30 whereby the end plug 50 may be releaseably mounted to the body 30 by screwing the set of threads 51 into the second set of threads 33 on the body 30. Referring now in particular to FIGS. 3 and 4, the end plug 50 has a body 54 having a tip 58 that extends axially therefrom and a central bore 55 that extends axially through the body 54 and tip 58, the portion of the bore 55 extending through the body 54 being provided with threads 53. The tip 58 of the end plug 50 may have a plurality of circumferentially spaced, axially extending slits 68 provided therein for providing air communication passages between the upper portion of central bore 55 and central bore 32. The high pressure stop 52 has a tip 56 that extends axially inwardly into the bore 55 of the end plug 50. The tip 56 of the high pressure stop has threads 57 compatible with the threads 53 on the central bore 55 through the end plug 50 whereby the high pressure stop 52 may be threaded into the end plug 50. Advantageously, the high pressure stop 52 may be selectively axially adjustable within the central bore 55 by turning the head 98 at the end of the body 54 of the high pressure stop 52 either clockwise or counter clockwise thereby selectively positioning the end face 59 of tip 56 of the high pressure stop 52 within the bore 55 of the end plug 50.

As noted hereinbefore, the axially elongated bore 32 of the body 30 of the valve 10 includes a central chamber 35 disposed between a pair of axially spaced, relatively larger diameter, generally cylindrical, end cavities 37 and 39, as best seen in FIG. 5. The central cavity 35 includes a relatively smaller diameter cylindrical section 63 and a conical section 65 that forms a transition between the relatively larger diameter end section 37 and the relatively smaller diameter section 63 of the central cavity 35. A first port 60 extends through the body 30 to open into the end cavity 37. A second port 62 extends through the body 30 to open into section 63 of the central cavity 35. The conical transition section 65 of the central cavity 35 lies intermediate the first port 60 and the second port 62 and tapers inwardly from cavity 37 to section 63 of the central cavity 35 at a angle of approximately ten degrees relative to the axis of the bore 32 through the valve body 30. However, one skilled in the art will understand that other angles of taper may be used without departing from the teachings of the invention.

Referring now to FIGS. 5 and 6 in particular, the valve 10 further includes a piston 70 adapted to be disposed within the axially elongated bore 32 of the body 30 and operatively associated with the body 30 for axial translation within the axially elongated bore 32. The piston 70 includes a head 72, which may have one or more holes 99 passing therethrough to reduce the overall weight of the piston 70, and a shaft 76. The generally cylindrical shaft 76 extends axially from a distal end of the head 72 of the piston 70 and into the bore 55 in the tip 58 of the end cap 50. The head 72 includes a plurality of guide members 74 extending radially outwardly from a proximal end of the head 72. In this manner, the piston 70 is supported at its opposite ends with the guide members 74 bearing on the inner wall of cavity 37 of the central bore 32 at its proximal end and with the shaft 76 bearing on the inner wall of the tip 58 of the end cap 50 at its distal end. The wide spacing between these bearing surfaces ensures stability in translation of the piston 70 within the central bore 32 of the body 30 of the valve 10 and decreased sensitivity to variation in tolerances which could otherwise result in rocking. Further, the circumferentially spaced slits 68 in the tip 58 of the high pressure stop serve as vent channels for venting flow as the piston translates within the bore 32, thereby preventing pressure build-up that could otherwise hinder proper translation of the piston. The flow passages 88 between the circumferentially spaced guide members 74 of the piston 70 ensure that the entire cavity 37 of the valve body has the same air pressure.

A spring member 80 is disposed about the shaft 76 between the distal end of the body 72 and the end face 78 of the tip 58 of the end cap 50 which serves as a stop for the spring member 80. The spring member 80 may be a resilient compression coil spring, such as shown in the depicted embodiment, or other type of resilient member capable of compression and expansion.

Referring again now to FIG. 1, the first port 60, which opens to cavity 37 of the central bore 32 in the body 30 of the bleed valve 10, is adapted to receive fluid flow from a conduit 100 that is in flow communication with a volume of fluid under pressure, for example a blood pressure measurement cuff (not shown). A fitting 102 may be provided, either as a separate component or as an integral part of the valve body 32, as a means for connecting the conduit 100 to the first port 60. In the depicted embodiment, the conduit 100 comprises a tube connected to a blood pressure cuff or blood pressure gage of a blood pressure measurement device (not shown), and the fitting 102 comprises a conventional threaded fitting of the type commonly employed with blood pressure tubing. The fitting 102 has a threaded stem which is screwed into threads provided in the port 60 and a tapered nipple extending outwardly therefrom and adapted to be received in sealing engagement into the conduit 100. The second port 62, which opens to the relatively smaller diameter cylindrical section 63 of the central cavity 35, serves as a vent port through which fluid flowing into the bleed valve 10 through the first port 60 may be vented to a low pressure, typically ambient pressure, environment exterior of the valve 10.

In operation, the first port 60 lies upstream of the conical transition section 65 of the central cavity 35 with respect to fluid flow through the central bore 32 of the valve 10 and the second port 62 lies downstream of the conical transition section 65 of the central cavity 35 with respect to fluid flow through the central bore 32 of the valve body 30. In accord with one aspect of the invention, a circumferential ridge 75 is provided on the piston 72 as a means of throttling fluid flow through the central bore 32 of the valve body 30. In the embodiment depicted in FIG. 7, the circumferential ridge 75 has a radially outward surface 77 extending about the circumference thereof that is axially tapered inwardly from its upstream end to its downstream end at an angle of approximately ten degrees relative to the axis of the bore 32 through the valve body 30. However, one skilled in the art will understand that other angles of taper may be used without departing from the teachings of the invention. Those skilled in the art will also understand that the outer surface of circumferential ridge is not limited to a tapered surface, but rather may take other shapes, including for example planar, arcuate or any other desired shape, that in operative association with the valve body provides an annular orifice defining a flow area that is variable with axial translation of the piston 70.

The circumferential ridge 75 is disposed within the conical transition section 65 of the central cavity 35 and translates axially within the conical transition section 65 as the piston translates. In cooperation, the tapered wall 67, best seen in FIG. 7, of the valve body 30 within the conical transition section 65 and the tapered surface 77 of the circumferential ridge 75 on the piston 72 form an annular orifice 85 therebetween. As the piston 70 translates axially within the central bore 32 of the valve body 30 under the influence of varying pressure in the cavity 37, the flow area provided through the annular orifice 85 will change, becoming smaller as the piston 72 translates axially toward the high pressure stop 52 and becoming larger as the piston 72 translates axially toward the low pressure stop 42. All throttling of the flow occurs at this orifice which is always positioned between the bearing surfaces of the piston within the valve body.

When the valve 10 is connected to a blood pressure measurement device, or other reservoir of pressurized fluid, typically air, fluid flow will enter the cavity 37 through the first port 60 and establish a pressure within the cavity 37 that will be approximately equal to the cuff pressure. The fluid flow entering through the first port 60 will fill the cavity 37 and exert a force on the end face 71 of the piston 72 that will act against the force exerted by the coil spring 80 on the piston 72. The force exerted by the fluid pressure within the cavity 37 upon the piston 72 will cause the piston to translate axially towards the high pressure stop 52. As the piston translates toward the high pressure stop 52, as illustrated by position A in FIG. 7, the annular orifice 85 established between the tapered surface 77 of the circumferential ridge 75 on the piston 72 and the tapered wall 67 of the valve body 30 will narrow, thereby resulting in a decrease in the flow area through the annular orifice 85. As the pressure within the cavity increases with an increase in cuff pressure, the piston 72 will continue to translate axially toward the high pressure stop 52 until either an equilibrium of the respective forces due to air pressure and the spring pressure occurs or the end face 79 of the piston shaft 76 abuts against the end face 59 on the tip 56 of the high pressure stop 52. The high pressure stop 52 can be adjusted by turning the head 58 of the high pressure stop 52 counter-clockwise or clockwise to selectively position the end face 59 of the tip 56 of the high pressure stop within the cavity 39, thereby selectively limiting the travel of the piston 72 to ensure that the flow area provided by the annular orifice 85 does not decrease below a desired lower limit. In this manner, the high pressure stop 52 can be set to ensure that bleed flow can not be completely shut off, thereby protecting against the cuff being inflated to an excessive pressure.

Conversely, as the pressure within the cavity 37 decreases, for example during deflation of the cuff, the piston translates toward the low pressure stop 42, as illustrated by position B in FIG. 7. The annular orifice 85 established between the tapered surface 77 of the circumferential ridge 75 on the piston 72 and the tapered wall 67 of the valve body 30 will enlarge, thereby resulting in an increase in the flow area through the annular orifice 85. As the pressure within the cavity decreases with a decrease in cuff pressure, the piston 72 will continue to translate axially toward the low pressure stop 42 until an equilibrium of the respective forces due to air pressure and the spring pressure occurs or the end face 71 of the piston 72 contacts against the end of the tip 48 of the low pressure stop 42. The low pressure stop 42 can be adjusted by turning the head 44 of the low pressure stop 42 counter-clockwise or clockwise to selectively position the end of the tip 48 of the low pressure stop within the cavity 37, thereby selectively limiting the travel of the piston 72 to ensure that the flow area provided by the annular orifice 85 does not increase above a desired upper limit. With the high and low pressure stops properly adjusted, the tapered surface 77 on the circumferential ridge 75 on the piston 72 and the tapered wall 67 of the valve body 30 will never contact thereby ensuring no wear of the piston or valve in the throttling region. Further, because these surfaces will not contact, flow will always be able pass through the orifice 85 to “clean” this region and preclude potential build-up of dust or other debris in this region.

The flow rate of fluid flow passing through the annular orifice 85 will change as a function of the flow area provided by the annular orifice 85. When the pressure within the cavity 37 is high, the flow area provided by the annular orifice 85 is relatively small. Conversely, when the pressure within the cavity 37 is low, the flow area provided by the annular orifice 85 is relatively large. As the pressure within the cavity 37 decreases during fluid passage through the annular orifice from the cavity 37 into section 63 of the central cavity 35 and out the vent port 62, the flow area at the annular orifice 85 increases, thereby resulting in an increase in the bleed flow rate and thereby also tending to maintain a constant cuff pressure decrease rate. Therefore, the bleed flow valve 10 is self-compensating in that the flow area changes inversely with the pressure within the cavity 37 and therefore inversely to the cuff pressure or pressure within whatever reservoir to which the first port 60 is connected. The self-compensating characteristic of the bleed flow valve 10 of the invention enables the cuff or reservoir bleed pressure change rate to effectively be maintained at a relatively constant value over a major portion of the pressure bleeding process.

In normal operation, the pressure change rate is controlled by the equilibrium balancing between the opposing air pressure and spring forces on the piston. The low pressure stop 42 and the high pressure stop 52 do not control the cuff pressure change rate during normal operation, but rather serve to provide limits on the minimum cuff pressure change rate and maximum cuff pressure change rate. The spring member 80 and its preload are selected to provide a desired spring characteristic that will ensure a cuff pressure change rate within a specified range of desired cuff pressure change rates. End plug 50 serves to preload the spring 80. The preload on the spring 80 may be adjusted as desired simply by turning the end plug 50 clockwise or counterclockwise. The preload on the spring 80, in conjunction with the spring geometry and the taper angles, determine the operational characteristic of the valve 10, that is the bleed pressure change rate of the cuff. The desired cuff pressure change rate can be set at the factory to the recommend rate or can be set or adjust by a skilled medical practitioner by adjusting the preload on the spring 80 as previously discussed.

The self-compensating characteristic of the bleed flow valve 10 of the invention offers a distinct advantage when used in connection with blood pressure measurement devices, such as manually operated sphygmomanometers and automated electronic blood pressure monitors. By proper positioning of the low pressure stop 42 and the high pressure stop 52, and by proper adjustment of the preload on the spring 80, the bleed pressure change rate during deflation of the blood pressure cuff may be constrained within desired minimum and maximum limits, or near the desired 2-3 mm Hg per second rate. In traditional manually operated sphygmomanometers, the user must continually adjust the valve as the pressure decreases to attempt to maintain a relatively constant cuff pressure change rate. Conventionally, automated electronic blood pressure monitors are provided with pressure transducers and an electronic circuitry that automatically controls and adjusts a bleed valve as pressure decreases so as to maintain a relatively constant pressure drop rate. The required pressure transducers, electronic circuitry and electronically-controlled bleed valve are expensive.

The bleed valve 10 of the present invention may advantageously be employed in operative association with an on/off vent valve, particularly in blood pressure measurement applications. For example, a conventional thumb screw valve, such as commonly included in association with the inflation bulb on manual blood pressure sphygmomanometers, may be operatively associated with the vent port 62 of the bleed valve 10 for enabling the user to selectively open or close the vent port 62. When inflating the blood pressure cuff, the user would position the thumb screw vent valve in a fully closed position. When the cuff has been inflated to the desired pressure level, the user would simply reposition the thumb screw valve to its fully open position and the bleed valve 10 would control the rate of deflation of the cuff as hereinbefore described to maintain a relatively constant cuff pressure change rate throughout the deflation process.

However, in blood pressure measurement applications, it may be desirable, for example for patient comfort, to rapidly deflate the cuff once the diastolic blood pressure measurement has been completed. Referring now to FIGS. 8A, 8B and 8C, the bleed valve 10 is coupled in operative association with a vent control valve 120 that is selectively positioned by the user amongst a first position, illustrated in FIG. 8A, wherein the vent port 62 of the bleed valve 10 is closed for inflation of the blood pressure cuff, a second position, illustrated in FIG. 8B, wherein venting of the cuff during deflation is controlled through the bleed valve 10, and a third position, illustrated in FIG. 8C, wherein the cuff is rapidly deflated directly through the vent control valve 120.

In the exemplary embodiment depicted in FIGS. 8A, 8B and 8C, the vent control valve 120 includes an axially elongated body 122 having an axially aligned central bore 124 extending therethrough and an axially elongated plunger rod 126 disposed within the central bore 124. The plunger rod 126 is adapted to be axially translatable within the central bore 124 among the aforementioned first, second and third positions. The valve body 122 is provided with a first port 121 at a proximal end of valve body, a second port 123 at a distal end of the valve body, and a third port 125 intermediate the first and second ports. Each of the ports 121, 123 and 125 extends generally radially through the valve body 122 to open into the central bore 124 thereby establishing a flow path through the wall of the valve body. The first port 121 is coupled to the conduit 100 via branch conduit 101. Thus, port 60 of the bleed valve 10 and the first port 121 of the vent control valve 120 are directly connected to the conduit 100, which in the depicted embodiment comprises a tube connected to a blood pressure cuff (not shown). The second port 123 of the vent control valve 120 is connected via conduit 104 directly to the second port 62, i.e. the vent port, of the bleed valve 10. The third port 125 of the vent control valve 120 serves as a vent port for venting the central bore 124 of the vent control valve 120 to a low pressure environment, typically to atmospheric pressure. Generally, the third port 125 may simply be open directly to the atmosphere.

The central bore 124 is sealed at the proximal and distal ends of the valve body 120 by means of seals 128 and 129, respectively. The seals 128 and 129 seal the gap between the axially translatable plunger rod 126 and wall of the valve body 120 defining the central bore 124. Each of the seals 128 and 129 may constitute an O-ring of conventional sealing material carried in circumferential glands provided in the wall bounding the central bore 124. Additionally, a pair of axially spaced ring seals 130 and 132 is carried in corresponding circumferential grooves on the plunger rod 126. As with the seals 128 and 129, the ring seals 130 and 132 also seal the gap between the axially translatable plunger rod 126 and wall of the valve body 120 defining the central bore 124. Therefore, three sealed cavities are established within the central bore 124 of the valve body 122 irrespective of the position of the plunger rod 126 within the central bore 124. A first cavity 133 is formed between the proximal seal ring 128 and the ring seal 130, a second cavity 135 is formed between the axially spaced seal rings 130 and 132, and a third cavity is formed between the ring seal 132 and the distal seal ring 129.

Referring now to FIG. 8A in particular, with the plunger rod 126 positioned as depicted in the first position within the valve body 122, the first port 121 opens to the first cavity 133, the second port 123 opens to the third cavity 137 and the third port 125 opens to the second cavity 135. Thus, in the first position, the vent port 62 of the bleed valve 10 is effectively closed in that the vent port 62 is coupled in flow communication through the conduit 104 and the second port 123 with the sealed cavity 137. Similarly, conduit 100 from the cuff is coupled in flow communication through branch conduit 101 and the first port 121 with the sealed cavity 133. The third port 125 opens to the sealed cavity 135. Therefore, the first position constitutes the “off” or closed position for the vent control valve 120 in that the vent port 62 of the bleed valve 10 is not in flow communication with the third port 125, i.e. the vent port, of the vent control valve 120. The vent control valve 120 is selectively positioned in this first position whenever it is desired to inflate the blood pressure cuff.

Referring now to FIG. 8B in particular, with the plunger rod 126 positioned as depicted in the second position within the valve body 122, the first port 121 again opens to the first cavity 133, the third port 125 again opens to the second cavity 135, but the second port 123 now also opens to the second cavity 135. Thus, in the second position, the vent port 62 of the bleed valve 10 is coupled in flow communication with the third port 125 of the valve body 122 through the conduit 104, the second port 123 and the second cavity 135. Conduit 100 from the cuff remains coupled in flow communication through branch conduit 101 and the first port 121 with the sealed cavity 133. Therefore, the second position constitutes the “on” or first open position for the vent control valve 120 in that the vent port 62 of the bleed valve 10 is now in flow communication with the third port 125, i.e. the vent port, of the vent control valve 120. The vent control valve 120 is selectively positioned in this second position whenever it is desired to deflate the blood pressure cuff in a controlled manner through the bleed valve 10.

Referring now to FIG. 8C in particular, with the plunger rod 126 positioned as depicted in the third position within the valve body 122, the first port 121 again opens to the first cavity 133, the second port 123 again opens to the second cavity 135, but the third port 125 now also opens to the first cavity 133. Thus, in the third position, the first port 121 of the valve body 10 is now coupled in flow communication with the third port 125 of the valve body 122 through the first cavity 133. Conduit 100 from the cuff is thus coupled in flow communication through branch conduit 101, the first port 121, and the sealed cavity 133 directly with the third port 125 of the vent control valve 120. Therefore, the third position constitutes the rapid deflate or dump position for the vent control valve 120 in that the conduit 100 from the blood pressure cuff is now in direct flow communication with the third port 125, i.e. the vent port, of the vent control valve 120. The vent control valve 120 is selectively positioned in this third position whenever it is desired to bypass the bleed valve 10 entirely and rapidly deflate the blood pressure cuff.

Referring to FIGS. 1 and 3, in particular, in the embodiment of the bleed valve 10 shown, a pair of seals, depicted as O-rings 95 and 97 carried in circumferential glands respectively on the end plug 50 and the high pressure stop 52, are provided to seal the cavity 39 thereby preventing leakage through the threaded connections between the end plug 50 and the distal end of the bleed valve body 30 and between the high pressure stop 52 and the end plug 50. With the seals 95 and 97 so disposed, all of the vented air must pass through the vent control valve 120. However, it is to be understood that the seals 95 and 97 are not needed if the bleed valve 10 is not operatively associated with a vent valve 120 to control the venting of air through the bleed valve 10.

In the embodiment of the bleed valve 10 of the invention depicted in FIGS. 1 through 7, the central bore 32 includes a conical transition section 65 that tapers inwardly from a relatively larger diameter cavity 37 upstream of the transition section to a relatively smaller diameter cavity 35 downstream of the transition section. The circumferential ridge 75 on the piston 70 in operative association with this conical transition section provides an annular orifice 85 that defines a flow area that varies in response to axial translation of the piston 70. It is to be understood that the invention is not limited to that particular embodiment. Rather, those skilled in the art will understand that other respective configurations of the outer circumferential surface of the piston 70 and the transition section in the central bore 30 may effectively cooperate to provide an annular orifice defining a flow area that varies in response to axial translation of the piston 70 within the central bore 32 of the valve body 30.

For example, referring now to FIG. 9, an alternate embodiment of the invention is depicted wherein the central bore 32 within the valve body 30 of the bleed valve includes a step transition from a relatively larger diameter section 37 and a relatively smaller diameter section 35 and the piston 70 has a tapered surface 77 that extends between and tapers inwardly from the larger diameter piston head 72 to the smaller diameter piston shaft 76. In this embodiment, the annular orifice 85 is established between the tapering surface 77 on the piston 70 and the radially inner corner 88 provided by the step transition in the central bore 32. The annular orifice 85 so provided defines a flow area that will vary upon axial translation of the piston 70. The flow area defined by the annular orifice 85 will increase in area as the piston 70 translates axially in the direction indicated by arrow A, being biased in that direction by the spring 80 as the pressure in the blood pressure cuff, or other pressure reservoir, decreases as air vents therefrom. Conversely, the flow area defined by the annular orifice 85 will decrease in area as the piston translates axially in the direction indicated by arrow B, being biased in that direction as the pressure in cavity 37 increases in response to an increase in the pressure within the blood pressure cuff or other pressure reservoir. Therefore, the embodiment depicted in FIG. 9 will also operate in accord with the invention to automatically vary the flow area defined by the annular orifice 85 in response to a change in pressure of the air within the upstream cavity so as to maintain a relatively constant pressure change rate throughout the venting process.

As noted before, the bleed valve 10 is particularly useful in blood pressure measurement applications. For example, in a typical blood pressure measurement procedure, the user, whether a physician, nurse, EMT, other trained professional, or the patient, first connects the measurement cuff to the pump of an automated electronic measurement apparatus or the inflation bulb of a manual sphygmomanometer, if not already connected, and then wraps the cuff about the patient's arm as in conventional practice. The user then closes the vent port 62, for example by closing a thumb screw or, if the bleed valve 10 is connected to a vent control valve, such as the plunger type vent control valve 120, position the vent control valve in its closed (first) position. With the vent port 62 closed, the cuff is inflated with the pump or the sphygmomanometer bulb to a pressure somewhat above, for example approximately 30 millimeters Hg, the expected systolic pressure. Once the cuff is inflated to the desired pressure, the user opens the vent port 62, either by opening a thumb screw or, if the bleed valve 10 is connected to a vent control valve, such as the plunger type vent control valve 120, positions the vent control valve in its open (second) position. With the vent port 62 now open to atmospheric pressure, the cuff will deflate through the bleed valve 10 in a controlled manner at the desired pressure decrease rate, for example at the American Heart Association recommended rate of approximately 2-3 mm Hg per heartbeat. As the bleed valve 10 is self-compensating for cuff pressure, the piston 70 will self-adjust the area of the orifice 85 within the bleed valve 10 to maintain the rate of decrease in cuff pressure relatively constant as the cuff deflates through the bleed valve 10. Once the diastolic blood pressure reading has been obtained the cuff will continue to deflate at the controlled rate. However, if the bleed valve 10 is coupled to a vent control valve having a third position, such as the vent control valve 120, wherein the bleed valve 10 can be bypassed to permit a direct venting of the cuff to atmosphere, the user may selectively reposition the vent control valve to its rapid deflate position and proceed to rapidly deflate the cuff.

It is contemplated that the bleed valve 10 will be factory-calibrated during manufacturing to provide a relatively constant pressure decrease rate at the aforementioned AHA recommend rate for adult blood pressure measurement when a typical adult cuff is used. The pressure decrease rate, as well as other operational characteristics of the bleed valve 10, are determined by the spring constant of the spring 80, by the preload on the spring, by the specific valve geometry (for example the taper angle for the conical transition section 65), and by the effective cross-sectional piston and valve orifice areas at the annular orifice 85. Those skilled in the art will recognize that the particular dimensions, spring selection, spring preload and other design factors may be selected to provide the desired operational characteristics. For the range of cuff sizes needed, that is from neonate cuffs to large adult cuffs, a proper selection of the spring, orifice area and piston area provide relative independence of the cuff pressure decrease rate relative to the cuff size range.

Since the bleed valve 10 operates in response to the pressure differential across the orifice, and not in response to the volume of air in the cuff, the operation of the bleed valve 10 provides a cuff pressure decrease rate that is theoretically dependent on cuff size. However, with proper selection of bleed valve components, this dependence has been found to be sufficiently small, so that the bleed valve 10 may be used with various size cuffs. Nevertheless, depending upon cuff size or the preference of the user, the bleed valve 10 may require field adjustment to provide a pressure change rate somewhat different than the factory-calibrated pressure change rate. For example, if the bleed valve is employed with a non-adult cuff or the adult patient has a heart beat elevated well above about sixty beats per minute, it may be desirable to adjust the rate of pressure decrease during deflation of the cuff through the bleed valve 10. The rate of pressure decrease may be adjusted by changing the preload on the spring 80. To do so, the user merely turns the end fitting clockwise or counter-clockwise as appropriate to further compress the spring 80 to increase the preload on the spring 80 or to lessen the compression of the spring 80 to decrease the preload on the spring 80.

While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail and design may be effected therein without departing from the spirit and scope of the invention as defined by the claims. 

1. A bleed valve for controlling the pressure change rate in a reservoir of fluid under pressure when venting fluid therefrom through said valve, said valve comprising: a valve body having a central bore extending axially therethrough, the central bore having a transition section transitioning from a relatively larger diameter upstream of the transition section to a relatively smaller diameter downstream of the transition section, a fluid inlet port opening through said valve body into the central bore upstream of the transition section, and a fluid outlet port opening through said value body into the central bore downstream of the transition section; a piston disposed within the central bore of said valve body, said piston having an outer circumferential surface facing the transition section of the central bore thereby establishing an annular orifice defining a flow area between the outer circumferential surface on said piston and the transition section of the central bore, said piston being axially translatable within the central bore of said valve body; a biasing device operatively associated with said piston for exerting a force on said piston, said biasing device acting to translate said piston in an upstream direction in opposition to a fluid pressure force on said piston acting to translate said piston in a downstream direction, whereby said piston self adjusts axially within the central bore to vary the flow area defined by the annular orifice so as to maintain a relatively constant pressure change rate in said reservoir.
 2. A bleed valve as recited in claim 1 wherein said biasing device comprises a spring member and a preload device, said preload device for compressing said spring member to establish an initial preload force of said spring on said piston, said spring acting to translate said piston in an upstream direction.
 3. A bleed valve as recited in claim 2 wherein: said spring member comprises a compressible coil spring disposed about a downstream portion of said piston; and said preload device comprises a closure member disposed within the central bore downstream of and abutting said spring member, said closure member being selectively axially positioned so as to adjust the preload force on said piston.
 4. A bleed valve as recited in claim 1 wherein the transition section comprises a step transition from the relatively larger diameter to the relatively smaller diameter.
 5. A bleed valve as recited in claim 4 wherein a portion of the outer circumferential surface of the piston faces the step transition section and has a tapered surface.
 6. A bleed valve as recited in claim 1 wherein the transition section comprises a tapered surface facing the outer circumferential surface of said piston.
 7. A bleed valve as recited in claim 6 wherein said piston has a circumferential ridge extending thereabout.
 8. A bleed valve as recited in claim 7 wherein the circumferential ridge has a tapered outer circumferential surface.
 9. A bleed valve as recited in claim 1 further comprising a first closure member closing a first end of said valve body and a second closure member closing a second end of said valve body.
 10. A bleed valve as recited in claim 9 further comprising a low pressure stop supported by said first closure member, said low pressure stop being selectively axially translatable.
 11. A bleed valve as recited in claim 10 wherein said first closure member comprises an end cap.
 12. A bleed valve as recited in claim 9 further comprising a high pressure stop supported by said second closure member, said high pressure stop being selectively axially translatable.
 13. A bleed valve as recited in claim 12 wherein said second closure member comprises an end plug.
 14. A bleed valve as recited in claim 12 wherein said high pressure stop limits translation of said piston so as to prevent the outer circumferential surface of said piston from contacting the transition section of the central bore.
 15. A bleed valve for controlling the rate of change of pressure in an inflated blood pressure cuff when venting air from the blood pressure cuff to deflate the blood pressure cuff, said valve comprising: an axially elongated valve body having a first end and a second end and having a central bore extending axially therethrough, the central bore having a first generally cylindrical cavity, a second generally cylindrical cavity, and third cavity disposed between the first and second cavities, said third cavity having a transition section; a first closure member closing the first end of said valve body; a second closure member closing the second end of said valve body; a piston disposed within the central bore of said valve body, said piston having a generally cylindrical body including an outer circumferential surface on said valve body facing the transition section of the central bore thereby establishing an annular orifice defining a flow area between the outer circumferential surface on said piston and the transition section of the central bore, said piston being axially translatable within the central bore of said valve body; a first port opening through said valve body into said first cavity upstream of said transition section, said first port pneumatically coupled to the blood pressure cuff; a second port opening through said valve body into said third cavity downstream of said transition section, said second port being a vent; and a biasing device operatively associated with said piston for exerting a force on said piston acting to translate said piston in an upstream direction in opposition to a fluid pressure force on said piston acting to translate said piston in a downstream direction, whereby said piston self adjusts axially within the central bore to vary the flow area defined by the annular orifice so as to maintain a relatively constant cuff pressure change rate.
 16. A bleed valve as recited in claim 15 wherein said third cavity of the central bore of said valve body has a tapered transition section.
 17. A bleed valve as recited in claim 15 wherein said piston has a tapered outer circumferential surface facing the transition section of the central bore.
 18. A bleed valve as recited in claim 15 wherein said biasing device comprises a spring member and a preload device, said preload device for compressing said spring member to establish an initial force of said spring on said piston, said spring acting to translate said piston in an upstream direction.
 19. A bleed valve as recited in claim 15 further comprising a low pressure stop supported by said first closure member, said low pressure stop being selectively axially translatable.
 20. A bleed valve as recited in claim 15 further comprising a high pressure stop supported by said second closure member, said high pressure stop being selectively axially translatable.
 21. A bleed valve as recited in claim 15 further comprising a vent control valve operatively associated with said second port of said bleed valve, said vent control valve being selectively positioned in a first position wherein bleed flow can not vent through said second port of said bleed valve to atmospheric pressure and a second position wherein bleed flow can vent through said second port of said bleed valve.
 22. A bleed valve as recited in claim 15 wherein said piston self adjusts axially within the central bore to vary the flow area defined by the annular orifice so as to maintain a relatively constant cuff pressure change rate substantially independently of the size of the blood pressure cuff.
 23. A bleed valve as recited in claim 15 further comprising a vent control valve having: a vent control valve body having a cavity therein, said vent control valve body having a first port opening to the cavity and in flow communication with said blood pressure cuff, a second port opening to the cavity and in flow communication with said second port of said bleed valve, and a third port opening to the cavity and venting to atmospheric pressure; and a member operatively associated with said vent control valve body, said member selectively positioned in a first position wherein neither of the first port nor the second port of said vent control valve is in flow communication with the third port of said vent control valve, in a second position wherein only the second port of said vent control valve is in flow communication with the third port of said vent control valve, and a third position wherein only the first port of said vent control valve is in flow communication with the third port of said vent control valve.
 24. A method for venting air from an inflated blood pressure cuff through a bleed valve at a relatively constant cuff pressure change rate comprising the steps of: providing an annular orifice between an upstream cavity and a downstream cavity in said bleed valve, the annular orifice providing a flow path having a variable flow area between the upstream cavity and the downstream cavity; passing air under pressure from the blood pressure cuff to the upstream cavity; venting air from the downstream cavity; and automatically varying the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity so as to maintain said relatively constant cuff pressure change rate.
 25. A method as recited in claim 24 further comprising the step of preventing an increase in the flow area of the flow path between the upstream cavity and the downstream cavity in response to a decrease in the pressure of the air within the upstream cavity to a predetermined maximum flow area at a predetermined relatively low pressure.
 26. A method as recited in claim 24 further comprising the step of preventing a decrease in the flow area of the flow path between the upstream cavity and the downstream cavity in response to an increase in the pressure of the air within the upstream cavity to a predetermined minimum flow area at a predetermined relatively high pressure.
 27. A method as recited in claim 24 wherein the step of varying the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity comprises increasing the flow area in response to a decrease in the pressure of the air within the upstream cavity and decreasing the flow area in response to an increase in the pressure of the air within the upstream cavity.
 28. A method as recited in claim 27 further comprising the step of: exerting a biasing force on a piston, said biasing force acting to translate the piston in an upstream direction in opposition to a fluid pressure force on the piston, the fluid pressure force acting to translate the piston in a downstream direction, whereby the piston self adjusts axially within the central bore to adjust the annular orifice so as to maintain said relatively constant cuff pressure change rate.
 29. A method as recited in claim 28 further comprising the step of: establishing an initial biasing force on the piston acting to translate the piston in an upstream direction, whereby a desired pressure change rate is preset.
 30. A method as recited in claim 28 further comprising the step of: adjusting the biasing force on the piston acting to translate the piston in an upstream direction, whereby a desired pressure change rate may be selectively set.
 31. A method as recited in claim 28 further comprising the step of: adjusting the biasing force on the piston acting to translate the piston in an upstream direction in response to substantially large changes in cuff size, whereby a desired pressure change rate may be selectively set.
 32. A method as recited in claim 28 further comprising the step of: adjusting the biasing force on the piston acting to translate the piston in an upstream direction in response to a heart rate of a patient, whereby a desired pressure change rate may be selectively set.
 33. A method as recited in claim 24 wherein the step of providing an annular orifice between an upstream cavity and a downstream cavity in said bleed valve comprises providing a valve body having a central bore extending axially therethrough and having a transition section and an axially translatable piston disposed within the central bore of the valve body, the piston having an outer circumferential surface facing the transition section of the central bore thereby defining the annular orifice.
 34. A method as recited in claim 33 wherein the step of providing an annular orifice between an upstream cavity and a downstream cavity in said bleed valve further comprises providing a valve body having a central bore having a tapered transition section.
 35. A method as recited in claim 33 wherein the step of providing an annular orifice between an upstream cavity and a downstream cavity in said bleed valve further comprises providing a piston having a tapered outer circumferential surface facing the transition section of the central bore.
 36. A method as recited in claim 33 further comprising the step of: exerting a biasing force on the piston acting to translate the piston in an upstream direction in opposition to a fluid pressure force on said piston acting to translate said piston in a downstream direction, whereby said piston self adjusts axially within the central bore to adjust the annular orifice so as to maintain a relatively constant cuff pressure change rate.
 37. A method as recited in claim 24 wherein automatically varying the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity so as to maintain said relatively constant cuff pressure change rate comprises automatically varying the flow area of the flow path between the upstream cavity and the downstream cavity in response to a change in the pressure of the air within the upstream cavity so as to maintain said relatively constant cuff pressure change rate substantially independently of blood pressure cuff size. 